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Good sports: Hamstring findings may help injured athletes stay healthy

Athletes who strain a hamstring could avoid re-injuring the muscle by participating in targeted physical therapies and improving their running mechanics, according to University of Wisconsin-Madison researchers.

Hamstring strains occur when muscle fibers tear at the junction of muscle and tendon. Such injuries often occur as athletes sprint during sports like track, soccer, football and baseball.

Computer model of musculoskeletal mechanics of a sprinter

A computer model of the musculoskeletal mechanics of a sprinter can help identify risk areas. (large image)

Most hamstring injuries only temporarily sideline athletes, who undergo rest and therapy while the muscle heals. However, an athlete who’s endured at least one hamstring strain is likely to experience repeat injuries, says Darryl Thelen, an associate professor of mechanical and biomedical engineering.

He’s trying to learn why — and in the process, discover ways athletes might prevent both initial hamstring injuries and re-injuries.

Darryl G. Thelen

Darryl G. Thelen (large image)

Combining magnetic resonance (MR) imaging, studies of sprinting biomechanics, and computer simulations, Thelen and graduate students Liz Chumanov and Amy Silder are learning more about how hamstring strains heal and why injuries may recur. “We are particularly interested in how the muscle remodels following injury,” says Thelen.

He and the students are working with Orthopedics and Rehabilitation Assistant Professor Bryan Heiderscheit, Radiology Professor Mike Tuite and UW Health physical therapist Marc Sherry to study high school athletes who have had one or more hamstring injuries and have been cleared to return to their sport.

Each athlete undergoes an MR, which helps the researchers quantify how much scar tissue is present at the site of the prior injury. Surprisingly, says Thelen, the MR results paint very distinct pictures of the injury site — even though the injury occurred months earlier. “We’ve learned that even individuals who are five or six months post-injury often have residual scar tissue at the musculotendon junction, which is the site of prior injury,” he says.

The researchers compare the MR findings with analyses of the athletes in action. In their biomechanics laboratory, they motion-capture athletes sprinting on a high-speed treadmill. “We can record their whole body motion, and then in the lab we can generate computer models of them sprinting,” says Thelen. “We can actually estimate the muscle mechanics while the individual is sprinting and assess when they’re at risk for injury.”

So far, his team has learned that athletes are most likely to injure a hamstring during the late-swing phase of sprinting, during which both feet are off the ground and the leg is extended. “That’s when the hamstring is loaded and stretched and seems to be most susceptible to injury,” says Thelen.

The researchers’ computer simulations enable them to estimate how much load the hamstrings are under and how much they’re stretched. In animal models, says Thelen, the mechanical strain a muscle is experiencing is a good predictor of injury potential.

Scar tissue (circled) from a hamstring injury

Scar tissue from a hamstring injury on the left leg appears black on an MRI (circled), even six months after healing. (large image)

Now the researchers can translate what they’ve learned about hamstring muscle mechanics into how best to rehabilitate the muscle after injury. Sherry and Thomas Best, an associate professor of biomedical engineering, family medicine and orthopedics and rehabilitation, have discovered that exercise programs that strengthen the core muscles — the abs and lower back — are related to fewer hamstring re-injuries. “Through our experiments and simulations, we’ve been able to show that these muscles can have a large influence on pelvic orientation, which affects hamstring stretch — and thus, presumably affects injury potential,” says Thelen.

The group now is studying whether early mobilization of the hamstring in a controlled way shortly after injury may help the muscle remodel in a way that reduces re-injury risk.

Funded by the NFL Charities and the Aircast Foundation, the group’s studies of how muscles heal after injury also are relevant to other muscles susceptible to injury, as well as tissues cut during surgical procedures, says Thelen.